E12 error and thermostat cannot control fan independently; fan runs only with heat/cool or not at all

Warning: Turn off power to the furnace/air handler before touching any wiring.

• Step 1 – Check fan settings:
  • In thermostat settings, verify the fan mode (Auto/On) and system type (gas, electric, heat pump).
  • For electric heat or some heat pumps, the thermostat may control the fan directly; for gas furnaces, the furnace often controls the fan.
• Step 2 – Inspect G wiring:
  • Ensure the G wire is connected at both the thermostat and furnace control board.
  • Look for breaks, loose connections, or shorts to other wires.
• Step 3 – Test fan circuit:
  • At the furnace board, temporarily jumper R to G (with power on) to see if the fan runs.
  • If the fan runs with the jumper but not via the thermostat, the thermostat’s G output is suspect.
• Step 4 – Measure G output:
  • With a fan call active, measure voltage between G and C at the thermostat; should be ~24VAC.
  • If not, the G driver on the thermostat is likely defective.
• Step 5 – Replace thermostat if G output is bad:
  • Replace the thermostat if E12 persists and wiring/furnace board are confirmed good.